High aspect ratio metal microstructures and method for preparing the same

ABSTRACT

High aspect ratio metal microstructures may be prepared by a method involving 
     (i) forming a layer of a photoresist on a substrate; 
     (ii) exposing the layer to actinic radiation in an imagewise manner and developing the exposed layer to obtain a surface which contains regions having no remaining photoresist and regions covered with photoresist; 
     (iii) metallizing the surface to form a layer of metal on the region of the surface having no remaining photoresist and on the sides of the regions of photoresist remaining on the surface; and 
     (iv) optionally, stripping the photoresist remaining on the surface. 
     Such microstructures are useful as electron emitters, anisotropic high dielectric interconnects, masks for x-ray photolithography, carriers for the controlled release of active agents, and ultramicroelectrode arrays.

This application is a Continuation of application Ser. No. 08/216,319,filed on Mar. 23. 1994, now abandoned which is a Division of applicationSer. No. 07/874,403 filed on Apr. 27, 1992, now U.S. Pat. No. 5,342,737,issued Aug. 30, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high aspect ratio metal microstructuresand methods for preparing such high aspect ratio metal microstructures.

2. Discussion of the Background

There are a variety of important military and industrial applicationsfor high resolution metal microstructures with good adhesion on avariety of technologically relevant surfaces. These includeinterconnects and vias in silicon based microcircuits as well as highresolution conductive paths on printed circuit boards, packaging,microwave transmitters and receivers.

These are also a variety of application for high resolution metalmicrostructures with high aspect ratios. These include electron beamsources which make possible the fabrication of high power microwavedevices, free electron lasers, projection electron beam lithographicsources and flat panel displays. Other applications for high resolutionhigh aspect ratio structures include x-ray masks, self shieldinginterconnects, controlled release microvials, microelectrodes andscanning tunneling electron tips.

Recently there has been a large amount of interest in vacuum fieldemission from arrays of sharp tips (T. Utsumi, IEEE Trans. ElctronDevices, vol. 38, p. 2276 (1991)). This is because of their potentialuse in vacuum microelectronics (C. A. Spindt, et al, J. Appl, Phys.,vol. 47, p. 5248 (1976)), flat panel displays (Fortune, Dec. 2, 1991, p.132) high power switches (C. W. Roberson, Proc. Soc. Photo-Opt. Int.Eng., vol. 453, p. 320 (1983)), etc. Another geometry has recently beenreported that makes use of metal-coated biologically derived cylindersfor free electron laser applications (Kirkpatrick et al, Applied Phys.Lett., vol. 60, p. 1556 (1992); and Kirkpatrick et al, NuclearInstruments and Methods in Phys. Res., Elsevier, N.Y., p.1, (1991)).Numerical modeling has shown that electron beam brightness for a hollowcylinder geometry should be superior to that of a sharp tip. However,problems with the biologically derived cylinders fabrication processseverely limits their performance.

The Fowler-Nordheim, (Fowler et al, Proc. R. Soc. London A, vol. 119,p-173 (1928)) field emission current density, J_(FN), describes theprocess of quantum filed emission from a one-dimensional cold-cathodesystem, ##EQU1## where A=1.54×10⁻⁶, B=6.87×10⁷, y=3.79×10⁻⁴ (βE)^(1/2)/φ), t² (y)=1.1, v(y)=0.95-y², β is the field enhancement factor due tolocal geometry, E is the applied electric field in V/cm, and φ is thework function in ΘV of the surface emission material. Precise values fort² (y) and v(y) are reported in the literature (Miller, J. FranklinInst., vol. 282, p. 382 (1966), and Miller, J. Franklin Inst., vol. 287,p. 347 (1969)).

Hollow cylinders can be used to produce a local enhancement of theapplied electric field whose magnitude is dependent upon cylinderheight, the average spacing between nearest neighbors, the radius ofcurvature of the metal wall at the edge of the exposed hollow cylinder,and the nature of the surface near the exposed edge. Detailed numericalsimulations (Kirkpatrick et al, Nuclear Instruments and Methods in Phys.Res., Elsevier, N.Y. p.1, (1991)) of the electrostatic field in thevicinity of a hollow cylindrical structure have shown that fieldenhancement factors in the range β=150-250 may be readily achieved witha cylinder of diameter 0.5 μm and a height h=10-15 μm. The enhancementfactor may be increased an additional 2- to 4-fold by the inherentsurface roughness of the elctrolessly deposited metal film that makes upthe outer cylinder surface, yielding an enhancement factor in the rangeβ=300-1000.

The hollow nature of the protruding tubule microstructures also providesan electrostatic lensing effect for the emitted electrons: the thinnerthe tubule wall, the greater the self-focusing effect of the structure,and the more collimated the emitted bean. For suitably fabricatedstructures, with thin wall thicknesses near the emission tip, normalizedelectron beam brightnesses well in excess of 10⁶ A/cm² -rad² can beachieved (Miller, J. Franklin Inst., vol 287, p.347 (1969)).

However, to date no approaches have proven suitable for producing fieldemitter arrays (FEAs) with the above structural and functionalcharacteristics. In addition, no technique for the production of highaspect ratio metal microstructures has been demonstrated that has theability to precisely control the height, diameter, center to centerspacing, alignment, metal type, and metal thickness required for thesedevices.

In addition, it is desirable to use x-rays as the source of actinicradiation in photolithographic techniques, because the short wavelengthsof such radiation provides increased resolution. To fully realize theincreased resolution potentially afforded by x-ray photolithography,masks which contain structural details on the order of about 10 to 0.01μm in width are desired. Further to permit the use of such penetratingirradiation, masks with a stopping power equivalent to about 1-3 μm of ametal such as gold or nickel are required. However, to date masks whichcombine the desired fineness of structural detail and the requiredstopping power have not been available. High aspect ratio metalmicrostructures having heights of about 1 to 3 μm and structural detailshaving widths of 1 to 0.01 μm, if available, would thus be useful asmasks for x-ray photolithography.

Recently, the use of microtubules as carriers for the controlled releaseof active compounds, such as antifouling agents, pesticides,antibiotics, etc., has been reported. Thus, high aspect ratio metalmicrostructures which are in the form of tubules having one or two openends would be useful as carriers for the controlled release of activeagents. However, the production of such microtubules remainsproblematic, especially for those with only one open end.

In addition, the ability to prepare anisotropic high dielectricinterconnects between one or more microcircuits on separate layers of asemiconductor or similar device with the desired packing of today'sdense microcirucuits remains an elusive goal. If it were possible toprecisely, place metal structures with heights of 0.5 to 5 μm and widthsof 0.5 to ˜4 μm on the surface of a device, such metal structures couldserve as highly anisotropic interconnects between a circuit on the layeron which the structures are placed and a circuit on a layer subsequentlyadded with an increase in shielding of the electron field between twodifferent circuits in close proximity. For a general discussion of metalfeatures and pillars in multilevel interconnect metallization, see VivekD. Kulkarni et al, J. Electrochem. Soc. Solide State Sci. and Tech.,vol. 135, no. 12, pp. 3094-98, (1988).

It is also desirable to provide ultramicroelectrode arrays (UMAs) foruse as sensors, e.g., in solid state electrochemistry. The benefitsattending such electrodes have been discussed by Fleischmann et al, J.Phys. Chem., vol. 89, pp. 5537-5541 (1985). In particular, it is desiredto prepare addressable ring microelectrodes with metal line widthdimensions on the order of 500 to 1000 Å. However, the production ofsuch devices is currently unachievable. Penner et al, Anal. Chem., vol.59, pp. 2625-2630 (1987) report the production of ensembles ofultramicroelectrodes. However, the electrodes are neither regularlyspaced nor arranged substantially parallel to one another. Thus, if itwere possible to prepare regularly spaced addressable metalmicrostructures with such dimensions, such metal microstructures couldserve as UMAs.

Thus, there remains a need for high aspect ratio metal microstructureswhich would be useful as anisotropic interconnects, electron emitters,x-ray photolithography masks, ultramicroelectrode arrays, and carriersfor the controlled release of active agents. There also remains a needfor a method of producing such high aspect ratio metal microstructures.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide novelhigh aspect ratio metal microstructures.

It is another object of the present invention to provide a method forproducing such high aspect ratio metal microstructures.

It is another object of the present invention to provide novel electronemissive surfaces.

It is another object of the present invention to provide a method forproducing such electron emissive surfaces.

It is another object of the present invention to provide novel masks forx-ray photolithography.

It is another object of the present invention to provide novel carriersfor the controlled release of active compounds.

It is another object of the present invention to provide a method forpreparing such carriers for the controlled release of active compounds.

It is another object of the present invention to provide novelanisotropic high dielectric interconnects.

It is another object of the present invention to provide devices whichcontain anisotropic high dielectric interconnects.

It is another object of the present invention to provide a method forproducing such interconnects and devices containing such interconnects.

It is another object of the present invention to provideultramicroelectrode arrays.

It is another object of the present invention to provide a method forpreparing such ultramicroelectrode arrays.

These and other objects, which will become apparent in the course of thefollowing detailed description, have been achieved by the inventors'discovery that high aspect ratio metal microstructures may be preparedby a process involving:

(i) forming a layer of a photoresist, which is amenable to the adhesionof electroless plating catalyst or can be treated such that the surfacepromotes the adhesion of the catalyst, on a substrate;

(ii) exposing the layer to actinic radiation in an imagewise manner anddeveloping the exposed layer to obtain a surface which contains regionshaving no remaining photoresist and regions covered with photoresist;

(iii) metallizing the surface to form a layer of metal on the region ofthe surface having no remaining photoresist and on the sides of theregions of photoresist remaining on the surface; and

(iv) optionally, stripping the photoresist remaining on the surface.

Thus, the present invention employs advanced optical lithography to formhigh aspect ratio posts on a substrate. Metallization of the posts isaccomplished by use of a selective micro-metallization process. Thedimensions of the structures that can be produced by this approach aredetermined by the thickness of the resist or height of the resistmicrostructures, the width of the resist microstructures and thethickness of the metal overcoat. The cylinders that have been fabricatedusing this process are up to 24 microns in height and range from about 1to 13 microns in diameter. The wall thickness is controllable andexisting samples with ˜800 Å thickness have been produced (yielding anapproximate radius of curvature at the cylinder rim of 400 Å). Thevariability of the metal cylinder parameters appears to be well withinor exceed the acceptable tolerances for many of the applications listedabove.

It is also possible to metalize any resist microstructure having ageometry other than a cylinder, such as, e.g., squares, lines, ovals,etc.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same become betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 diagrams one embodiment of the present method as a flowsheet;

FIGS. 2a-d are scanning electron micrographs of high aspect ratio metalmicrostructures according to the present invention;

FIG. 3 graphically illustrates the relationship between mask featuresize and obtain metal microstructure height for mask dot featuresprinted on a Suss MJ3B vacuum contact printer.

FIGS. 4-6 are scanning electron micrographs of microcylinders accordingto the present invention;

FIG. 7 is a scanning electron micrograph of an array of microcylindersaccording to the present invention after field emission testing;

FIG. 8 is a schematic cross section of a preferred embodiment of thepresent microcylinders.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, the present invention, in one embodiment, relates to high aspectratio metal microstructures. By the term high aspect ratio is meantstructures in which the longest dimension is at least 1 times theshortest dimension. In one preferred embodiment the present metalmicrostructure is in the shape of a cylinder. In this case, it ispreferred that the greater dimension be the length of the cylinder andthat the lesser dimension be the outer diameter of the cylinder.Although the exact preferred dimensions of such cylinders will, in part,depend on the intended use of the cylinders, generally such cylinderswill have lengths ranging from about 1 to 30 μm and outer diametersranging from about 0.25 to 30 μm. Thus, the aspect ratio of suchcylinders is typically at least 1 to 30. Much higher aspect ratios havebeen achieved using conventional x-ray lithography techniques and thickx-ray resists. These structures can be prepared with nearly identicalbase and tip dimensions without any tapering and with heights as tall as200 μm.

The present microstructures may be in the shape of a hollow cylinderwith either one or two open ends. In this case, the length and outerdiameter of the cylinder are as described above, and the inner diameteris generally 0.25 to 30 μm and the cylinder wall thickness is typically500 to 10,000 Å.

The metal which comprises the present microstructures may be any thatmay be suitably employed in the present process of preparing the presentmicrostructures. Thus, the metal may be any that may be utilized in themetallizing step described below. Such metals and the correspondingplating baths are disclosed in U.S. Pat. No. 5,079,600; Science, v. 252,551(1990); Solid State Tech., vol. 34(10), p. 77(1991); Proc. MRS Spring1992 Mtg., Paper C8.22, and Proc. MRS Spring 1992 Mtg., paper C12.3, allof which are incorporated herein by reference. Further, the presentmicrostructures may include layers of metals subsequently applied to thefirst metal layer, by techniques such as vapor deposition and othertechniques discussed Thin Film Processes, Vossen, ed., Academic Press,N.Y. (1978). It should be understood that the final microstructure mayinclude layers of metal alloys which are formed by alloying two or moremetals which are layered in the same or different steps and alloyed by asubsequent procedure, such as a heat treatment. It should also beunderstood that the present metal microstructures include those whichcontain one or more metal oxide layers and/or small amounts of residualphotoresist or plating bath components. The microstructures may alsocontain small amounts of other materials arising from surfactants andother additives present in the resist.

As noted above, the present microstructures may be used as a carrier forthe controlled release of active agents. The use of microtubules ascarriers for the controlled release of active agents is described inU.S. patent application Ser. Nos. 07/343,762, filed on Apr. 14, 1989 and07/668,772, filed on Mar. 11, 1991, which are incorporated herein byreference. In this case, the microstructures are preferably hollowcylinders with either one or two open ends. Preferably these cylindershave a length of 1 to 30 μm, an outer diameter of 0.25 to 5 μm, an innerdiameter of 0.25 to 5 μm, and a wall thickness of 500 to 10,000 Å.

In another embodiment, the present microstructures may comprise anelectron emissive surface. In this case, the microstructures preferablycomprise an array of hollow microcylinders arranged on a conductivesurface such that the lengthwise dimension of each microcylinder isperpendicular to the conductive surface. Preferably, the microcylindersare 3 to 30 μm in height, 0.25 to 10 μm in outer diameter, 0.25 to 10 μmin inner diameter, 500 to 10,000 Å in wall thickness, and arranged suchthat the center to center distance for nearest neighbors is about 5 to50 μm, and all structures are of uniform height. Preferably, the presentelectron emissive device is structured such that there is alow-resistance path between the conductive substrate (power source) andthe microstructure tips. It has been found that this is achieved indevices which are substantially free of any high-resistance layer, suchas an oxide, including metal oxide layer, between the microcylinders andthe conductive substrate (power source). The presence of such a layermay lead to the destruction and ejection from the surface of themicrocylinders when a potential is applied. It should be understood thatthe presence of a high-resistance region (an oxide layer) per se in thepresent device may not necessarily lead to the destruction of thedevice. Thus, it is only when the high resistance region blocks a lowresistance pathway between the conductive surface (power source) and theemitting surface that destruction may occur. Accordingly, the presenceof a high resistance region may be tolerated or even desired in thepresent device so long as it does not preclude a low resistance pathwaybetween the conductive substrate (power source) and emitting surface.

The present electron emissive surfaces may be operated by applying apotential across the microcylinders as shown in FIG. 2. Typically,electron emission will be achieved with potentials ranging from 50 to200 KV/cm. Such potentials may be applied by conventional techniques asdescribed in, e.g., Kirkpatrick et al, Applied Phys. Lett., vol. 60, p.1556 (1992), and Kirkpatrick et al, Nuclear Instruments and Methods inPhys. Res., Elsevier, N.Y., p.1, (1991), both of which are incorporatedherein by reference. The microcylinders of the present electron emissivesurfaces may be addressed, in whole, in groups, or individually, bydesigning the mask used to fabricate the resist posts such that theposts are aligned with conductive pads or pathways that were previouslybuilt into the substrate using conventional IC fabrication techniques.

As noted above, the present electron emissive surfaces may be used indevices such as high power microwave devices, free electron lasers,projection electron beam lithographic sources, and flat panel displays.

In another embodiment, the present invention relates to masks for use inx-ray photolithography. In this case, the final geometry of the highaspect ratio microstructure will be determined by the desired mask imageto be utilized in the subsequent x-ray photolithography. Thus, there areno particular constraints placed on the pattern of the present x-rayphotolithography masks. It is, however, preferred that the metalmicrostructure have a height sufficient to provide a stopping powerequivalent to 1 to 3 μm of a metal, such as gold. Further, it is alsopreferred that the metal microstructures possess structural detailshaving widths of 0.01 to 1.0 μm to fully realize the potential highresolution afforded by x-ray photolithography. X-ray masks are typicallymade on an x-ray transparent membrane such as silicon nitride with highaspect ratio gold microstructures. The mask is placed in proximity of(˜25μ) a substrate coated with an x-ray resist and then is exposed tocollimated x-rays from a suitable x-ray source (see: PhotoreactivePolymers, pg. 335-358).

The high aspect ratio metal features block the x-rays in a pattern thatis transferred to the resist. The resolution of this pattern is dictatedby the width of the metal microstructures. Maintaining the width of thestructure from the base to the top is crucial. Making 1-3 μm high metalstructures with widths of 0.5-0.01 μm has proven very difficult.

With the process described in this patent a 1μ wide, 3 82 m thick photo,electron beam, ion beam, or x-ray resist may be electrolessly platedwith a metal of a known thickness by controlling the plating conditions.For example, if the resist was plated with 500 Å of NiB the surface ofthe substrate and the side walls of the of the resist will be plated tothat thickness. The 500 Å on the surface of the substrate will haveminimal x-ray blocking capability. However, the 500 521 of metal on theresist sidewalls will have the stopping equivalent of 3 μm of metal.Producing this structure on a suitable x-ray transparent membranematerial and then using it as an x-ray mask would result in parallelbeams 1 μm apart that are 500 Å in width in conventional x-ray resists.

The present invention also relates to anisotropic interconnects whichare useful for connecting a first circuit on one layer of asemiconductor or similar device with a second circuit on a differentlayer. The present invention also relates to devices containing suchinterconnects. In this case it is preferred that the interconnect be ahigh aspect metal microstructure in the form of a metal post attached toa first surface which contains a first circuit. Of course, there may bemore than one interconnect attached to the first surface. In the presentdevices, the volume between the interconnects can be filled with asuitable nonconductive material. Such materials include polyimides, forexample. The device also contains at least one additional circuit whichis on the surface formed by the top of the interconnect(s) and the topof the layer of nonconductive material residing between theinterconnects. The precise relative positioning of the first circuit,the interconnects, and the second circuit may be easily achieved byconvention photolithographic layer-to-layer alignment techniques.

In another embodiment, the present invention provides a method forpreparing the present high aspect ratio metal microstructures. Inparticular, the present method involves:

(i) forming a layer of a photoresist on a substrate;

(ii) exposing the layer to actinic radiation in an imagewise manner anddeveloping the exposed layer to obtain a surface which contains regionshaving no remaining photoresist and regions covered with photoresist;

(iii) metallizing the surface to form a layer of metal on the region ofthe surface having no remaining photoresist and on the sides of theregions of photoresist remaining on the surface; and

(iv) stripping the photoresist remaining on the surface.

In the first step a layer of photoresist is formed on a support. Thesupport may be any of a great number of materials, and the selection ofthe support material will depend, in part, on the shape and intended useof the final microstructure. For example, when the microstructure beingprepared is a hollow cylinder to be used as a carrier for the controlledrelease of an active agent, the only requirement placed upon the supportis that it be able to accept and bind a layer of a photoresist. Suchsupports include those composed of stainless steel, copper, silicon,gold, Ti/Ni/Au, GaAs, diamond, plastics i.e., ABS, crosslinkedpolyvinylphenol resins, polyethylene. Some of these substrates mayrequire the use of an adhesion promoter for adhesion of the photoresistto the substrate. Examples of such an adhesion promoter are(aminoethylaminomethyl)phenethyl-trimethoxysilane andhexamethyldisilazane. It may also be necessary to treat exposed areas ofthe substrate to promote metallization of these areas if metallizationof these areas is desired. This may be accomplished by the methoddisclosed in U.S. Pat. No. 5,079,600 and U.S. patent application Ser.No. 07/691,565, both incorporated herein by reference.

In contrast, when the microstructures being prepared are to be used asan array for electron emission, it may be preferable to use as thesupport a highly conductive non-oxidizing metal such as gold orplatinum.

Similarly, the photoresist may be any of a great number of compounds, solong as it is capable forming a thin layer on the support, is capable ofbeing patterned by conventional photolithography, electron beamphotolithography, ion beam photolithography, x-rays, etc., and iscapable of being selectively metallized, on the surfaces which have beenexposed as a result of lithography. Additionally, photoresists which arecapable of being metallized even on surfaces which have not been exposedas a result of photolithography, may be utilized in certainapplications, notably the production of hollow cylinders with one openend. For this application a more robust metallization scheme may berequired, such as increased catalyst concentration, contact time, metalbath concentration, and/or temperature.

Practically all resists are capable of being selectively metallized onsurfaces exposed, as a result of photolithography, by controlling themetallization conditions. Preferred resists include SAL601®, S1400®, SNR248®, S 1650® (all products of Shipley Co., Inc. Newtown Mass.) AZ4620®, AZ 4400®, and AZ 4903® (all products of Hoescht, Germany). Theseresists include the novolak based and polyvinylphenol based resins withdiazoquinone and acid catalyzed photoactive compounds.

Resists capable of being metallized on all surfaces includepolymethylmethacrylate (PMMA) and the resists described above when morerobust metallization conditions are used. For a comprehensive list ofsuitable resists see: Photoreactive Polymers The Science and Technologyof Resists, A. Reiser, Wiley, N.Y. (1989).

As will be made more explicit below, the thickness of the photoresistlayer, in part, determines the long dimension of the presentmicrostructures. Thus, the thickness of the photoresist layer issuitably 200 to 1 μm, preferably 30 to 1 μm. The photoresist layer istypically formed by spinning a layer of the photoresist compound on thesupport, and the thickness of the photoresist layer may be convenientlycontrolled in a given thickness range by adjusting the speed and lengthof time of the spinning and by selecting the solids content of theresist.

The spin speed can be adjusted to vary the film thickness of a givenphotoresist. There is an optimal range of spin speeds that will givereproducible and uniform films (Elliot, Microlithography ProcessTechnology for IC Fabrication, McGraw Hill, N.Y, p.74-87, (1986)).

A multiple spin technique can be used to build thicker layers of a givenphotoresist than those allowed by the solids content and spin speedparameters. With the multiple spin technique a single layer ofphotoresist is spun on to the support. The layer of photoresist ispartially baked to harden the base layer so that it will not mix withsubsequent photoresist layers. After the bake cycle a second layer ofphotoresist is applied and spun on the support. Thicker layers of resistcan be built by continuing this spin-bake-spin cycle.

There are practical limits to this process. Defect generation of eachspin/bake cycle, photoresist edge build up after each spin and crackingand bubbling of the photoresist due to outgassing of the photoresistlayers during the bake cycle will effect film uniformity. A typicalthick photoresist such as AZ-4903 can be doubled in thickness with aminimum of side effects.

However, other coating techniques have been demonstrated that haveproduced 100-200 μm of PMMA for x-ray lithography.

After the photoresist layer has been formed, it is patterned byconventional photolithographic techniques. Thus, the photoresist isexposed to actinic radiation in an imagewise fashion by the use of amask or retical. It should be noted that the choice of pattern in themask will, at least in part, determine the final geometry of theresulting microstructures. When selective metallization of the surfacesexposed by photolithography of a positive resist is being employed, asin the production of an electron emissive surface, the areas masked inthe photolithography step will not be metallized, and the dimensions ofthe masked areas will directly translate into dimensions betweenmetallized areas. For example, when the photolithography mask is apattern of circular dots, the diameter of the dots will correspond tothe inner diameter of the final hollow microcylinders, e.g., in anelectron emissive surface. A mask containing holes could be used toachieve the same result with a negative resist.

After the photoresist layer has been exposed to actinic radiation, it isthen developed to remove a portion of the photoresist layer. Thisdevelopment step is carried out using conventional techniques andmaterials as described in Photoreactive Polymers The Science andTechnology of Resists, A. Reiser, Wiley, N.Y. (1989) which isincorporated herein by reference. This development step will result in asurface which contains regions from which the photoresist has beenremoved and regions on which the photoresist remains.

The next step involves metallizing the developed surface. A key featureof the present invention resides in the fact that the sides of theregions of the photoresist remaining on the substrate are metallized.Another key feature of the present invention resides in the fact thatthe thickness of the layer of metal formed on the sides of the regionsof the photoresist remaining on the substrate may be convenientlycontrolled by adjusting the conditions, e.g., time, temperature,reactant concentrations, of the metallizing step. Thus, it is possibleto construct layers of metal which have a height which is determined bythe height of the photoresist layer and a width which is controlled byadjusting the metallizing conditions. These metal layers will be formedalong the contours of the regions of photoresist remaining on thesubstrate. For example, if the mask used in the photolithographic stepis a pattern of circular dots, the regions of photoresist remainingafter development will be a corresponding pattern of circular columns orposts, and the metal microstructures formed on metallization will be acorresponding pattern of hollow cylinders in which each hollow cylinderof metal surrounds a separate post of photoresist.

It should be understood, that by appropriate choice of resist materialand careful control of metallization parameters it is possible toachieve metallization on the top surface of the photoresist remaining onthe substrate as well as the sides. Thus, when using a photoresist suchas PMMA the metallizing step will proceed with the formation of a layerof metal on the top surface of the photoresist. This approach isparticularly useful if a hollow cylinder with one open end and oneclosed end is desired. In contrast, when a resist such as S 1650®, AZ4620®, AZ 4406®, and AZ 4903® is employed, it is possible to achievemetallization on the sides of the remaining photoresist, withsubstantially no metallization on the top surface of the photoresist.This is accomplished primarily by controlling the metallizationconditions, i.e., length of contact time, concentration, andtemperature. This approach is useful for the production of arrays ofhollow cylinder for electron emissive devices or hollow cylinders withtwo open ends.

It may also be possible to control the degree of metallization on theexposed surfaces which are parallel to the top surface of thephotoresist but between the regions of photoresist remaining on thesubstrate. Thus, if the photolithography step results in the creation ofregions in which the surface of the substrate is exposed, then theability to achieve metallization in these regions will depend on theability of the substrate to be metallized. For typical substrates suchas gold, oxides treated with self assembly monolayers as described inU.S. Pat. No. 5,079,600, metallization will proceed in these regionswithout difficulty, resulting in the metal layers along the sides of thephotoresist being connected by a layer of metal on the surface of thesubstrate.

The metallizing step may be carried out using conventional materials andtechniques, such as those described in C. R. Shipley, Jr., Plating andSurface Finishing, vol. 71, p. 92 (1984), which is incorporated hereinby reference. The choice of metal deposited in this step will, in part,depend on the intended application of the final product. For electronemitter arrays, metals such as Ni, Pd, Cu are suitable, and gold, nickelwith gold overcoats, and copper with gold overcoat, and platinum arepreferred. Carriers for the controlled release of active agents,disclosed in U.S. patent application Ser. No. 07/668,772, incorporatedherein by reference, are preferred. For x-ray photolithography masks,Au, Cu, NiB, NiP, Permalloy, and Pd are suitable, and Au is preferred.For anisotropic interconnects, Cu, Au, NiB, NiP, and Pd suitable, and Cuand Au are preferred.

As noted above, the thickness of the metal layer deposited (the smallerdimension of the high aspect ratio microstructure) may be controlled byadjusting the conditions of the metallizing step. Thus, for a typicalmetallizing bath such as 10% by volume Niposit® 468 (Shipley) whichplates at a rate of ˜20 Å/min, the thickness of the layer may be variedfrom about 100 Å to about 250,000 Å by varying the time of immersion inthe metallizing bath from about 5 min to about 12,500 min, at atemperature of about 25°. Faster rates can be achieved by simplyincreasing the concetration and temperature of the plating bath. Thethickness of the metal layer arising from other metallizing solutionsmay be similarly controlled by adjusting the time and temperatureparameters. For particularly fast acting metallizing solutions, it maybe necessary to adjust the concentration of one or more reagents in thesolution. The thickness of the resulting metal layer may be readilyascertained by resort to scanning electron microscope spectroscopy orSloan-Dektak profilometer (see Solid State Tech., vol 34, p. 77 (1991)).The ability to control the thickness of the metal layer by adjusting theabove described parameters and the ability to determine the thickness ofthe resulting metal layer are within those possessed by one of ordinaryskill.

After the metallization step, the remaining photoresist may be removedby the use of a conventional stripping step. The precise nature of theagents and conditions employed in the stripping step will, of course,depend on the identity of the photoresist employed in the previoussteps. However, the suitable and preferred conditions required forstripping any given conventional photoresist are known to those skilledin the art, as evidenced by Microlithography Process Technology for ICFabrication, Elliot, McGraw Hill, NY (1986), which is incorporatedherein by reference.

The result of removing the remaining photoresist in the stripping stepis to form a surface on which the high aspect ratio metalmicrostructures remain on the substrate. For some applications, such aselectron emitters, x-ray photolithography masks, and anisotropicinterconnects, it will be desirable to leave the metal microstructuresintact on the substrate. For other applications, such as carriers forthe controlled release of active agents, it may be desirable to removethe metal microstructures (microtubules) from the substrate. The removalof the metal microstructures may be achieved by limiting metallizationbetween microstrucutes and then rinsing the structures off the surface.

The present method is particularly well suited for the production ofsubstrate-mounted devices for the controlled release of an active agent.Such arrays of substrate-mounted microvials would be useful inapplications such as substrates used for the topologically (transdermal)delivery of drugs or as substrates that could be immersed in solution orother environments which release an agent at a linear reate for anextended period of time. In addition, the present method is well suitedfor the production of controlled release devices (microcylinders) whichare closed at one end. Since it is expected that, when other parametersare the same, microcyliners with one closed end will release an agent atone half the rate at which microcylinders with two open ends release thesame agent, microcylinders with one closed end would be useful toprolong the effect of the active agent. Of course, microcylinder withone closed end could be used in conjunction with those having two openends.

Even in regard to the production of microcylinders with two ends, thepresent method offers advantages not found in the conventionalapproaches for producing microcylinders. Thus, conventional approachesfor producing microcylinders suffer from the drawback that themicrocylinders produced are not always uniform in length, outerdiameter, and inner diameter. In contrast, the present method benefitsfrom the precision offered by photolithographic techniques, and thus,the resulting microcylinders are highly uniform with regard to thelength, outer diameter, and inner diameter. Typically, batches of up toabout 730 million microcylinders may be prepared, and the dimensions ofthe microcylinders vary by less 1%. Further, it is also possible to holdthe variations in dimensions to less than about 2% even formicrocylinders prepared in different batches. Even when using spinningspeeds above 3K rpm, to ensure film homogeneity, it is possible to varythe photoresist film thickness between 5 to 15 μm.

FIGS. 1a-e provide a schematic representation of one embodiment of thepresent method. In FIG. 1a, a photolithographic mask 1 consisting of apattern of circular dots is used to irradiate a layer of photoresist 2on a gold base 3 on a silicon substrate 4. After development of theirradiated photoresist, a pattern of circular columns (posts) 5 ofphotoresist is obtained as shown in FIG. 1b. The metallizing step yieldsthe article depicted in FIG. 1c, in which the sides of the photoresistposts are now coated with a layer of metal 6. In this particularembodiment, the photoresist is chosen such that the surfaces exposed byphotolithography may be selectively metallized. Thus the tops of thephotoresist posts are not coated with layer of metal. FIG. 1dillustrates the product of the removal of the remaining photoresist bystripping. Now hollow microcylinders of metal 6 remain on the gold base3. The use of the device of FIG. 1d as an electron emitter is shown inFIG. 1e. A potential is applied using a potential source 7, andelectrons 8 are emitted from the hollow metal microcylinders 6.

Other features of the invention will become apparent in the course ofthe following descriptions of exemplary embodiments which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLES Example 1 Field Emitter Array (FEA) Fabrication

Thin film photoresists were fabricated from four different commerciallyavailable resists. The photoresists S1650 from Shipley Co. and AZ4400,AZ4620, and AZ4903 from Hoechst AG Corp., were spun on 3 inch antimonydoped silicon wafers on a vacuum chuck at varying speeds from 1-4K rpm.The thickness of the resists was measured by surface profilometry andoptical interferometry. The resist thickness may be controlled byvarying the spin speed. Another observation in this work was that theresist homogeneity decreases dramatically below spin speeds of 3K rpm,for the resists described above. To produce resist films withhomogeneous thicknesses of 10 microns or greater, single and multiple3-4K spins of AZ4903 and AZ4400 resists were carried out.

A cross section of the metal microcylinder fabrication process as shownschematically in FIG. 1. The process starts with a flat substrate suchas a silicon wafer or other optically flat substrate, onto which, aphotoresist of known thickness is spun as described above. A chrome onfused silica mask is then placed in contact with the resist in a deep UVSUSS MJ3B high pressure Hg/Xe lamp contact printer and exposed with thecorrect dosage of light to clear the exposed regions of the photoresistafter development with the recommended alkaline developer. Thedevelopment results in resist posts that are the height of the resistthickness (as shown in FIG. 1b) and their center to center spacing, tip,and base diameters should vary as a function of the mask feature spacingand size. As well as the exposure dosage and type of exposure tool used.

Wafers with the photoresist structures were treated with a 3% by volumeCataposit®44 (Shipley Co.) catalyst solution for 5 min, then rinsed with0.1M HCl. The catalyzed surfaces were then accelerated with a pH 1solution of Na₂ PdCl₄ 3H₂ O (83 mg/L) for 4 min (Gulla et al, EP90-105228.2, (1990)). The wafers were rinsed with dionized water andsubsequently immersed in Niposit®468 (Shipley) EL nickel-boron platingbath, diluted to 10% by volumne of full strength, at 25° C. for timesranging from 30-60 min. The deposition rate for this bath has been shownto be ˜20 Å/min. A visually homogeneous Ni deposit was produced over theentire surface of the wafer. After removal from the EL bath, the waferswere rinsed with dionized water and dried under N₂. This results inphotoresist posts plated with ˜800-1000 Å of NiB on the sidewalls andplating on the surface of the substrate but not on the top of thephotoresist as shown in FIG. 1c.

Following metallization, the photoresist is removed by placing thesubstrate in acetone for 1 minute followed by rinsing with water. Thisresults in hollow metal microcylinders perpendicular to the substratewith wall thicknesses that are equal to the plating thickness (see FIG.1d).

Further processing can be done on the metal microcylinder arrays such ascutting the substrate into individual arrays and then mounting them oncopper stubs with silver paint, annealing and plasma etching to reducesurface oxidation, and sputtering of low work function metals like gold,without damage to the microstructures.

Example 2 FEA Property Characterization

Lithographicly defined microstructures were fabricated in 10 microns ofAZ4620 and characterized by scanning electron microscopy (SEM). Themetallized microstructures fabricated under the conditions described inExperiment 1 are shown in FIGS. 3a-d. These microstructures appear to benon-cylindrical and to be totally metallized, including their tops. Theaverage height of the tallest microstructure arrays fabricated usingthis procedure was only 2.5 microns, demonstrating that the largestdiameter dot sizes (2 micron on 5 micron centers) on this mask are notlarge enough to fabricate posts all the way through a 10 micron thickphotoresist. Four more samples were fabricated using the proceduredescribed in Experiment 1, and these were characterized immediatelyafter metallization by x-ray photoelectron spectroscopy (XPS). The foursamples characterized showed no evidence of tin, tin oxide or palladiumand showed very little evidence of oxidized nickel at the surface. Othersamples of electroless nickel deposited on lithographicly definedphotoresists were characterized by XPS, and all samples have shown noevidence of tin, tin oxide, or palladium at the metal surface,demonstrating that these contaminates are not present at the metalsurface. However, the nickel oxide content was found to increase withtime at the surface. The amount of oxide at the surface can be reducedbut not eliminated if the structures are annealed at 300°-400° C. for 1hour, after plating.

A second generation array test mask was then fabricated. This mask is4×4 inches with 36 chip areas of 1 cm² each. Every chip contains 4-1 mm²arrays of dots of a known diameter on 15 micron centers. The dotdiameters start at 3 microns and increase in 1 micron increments to 13microns and then the chip patterns are repeated again. This test maskshas two major functions; first, it is designed so that the relationshipof the metallized microstructure size and shape as a function of maskdot size and resist thickness can be determined; and second, the chipsize and array dimensions are designed so that the chips can easily becut out of the wafer for surface characterization and testing of theirfield emission properties.

The height of microcylinder arrays fabricated on 3 inch n-type siliconwafers using the second generation test mask and 10 and 24 micron thickphotoresists as a function of mask feature size is shown in FIG. 3. Thefigure shows that mask dot sizes of 5 microns or greater are required toproduce structures 10 microns tall in 10 micron thick resist and thatmask dot sizes of 10 microns or greater are required to producestructures 24 microns tall in 24 micron thick resists. Tip diameters ofthe structures range from ˜1.5 micron in the 5 micron dot arrays to ˜10microns in the in the 13 micron dot arrays for a 10 micron thickphotoresist. Tip diameters range from ˜1-6 microns in the 24 micron tallstructures.

Microcylinder arrays plated with 800 angstroms of nickel boron werecharacterized by scanning Auger Electron Spectroscopy (SAES) todetermine the elemental composition of these microstructure arrays as afunction of processing. SAES shows that the top of the resist posts haveno nickel but do contain small amounts of tin and palladium and that thesurface of the metal microcylinder does not contain any tin, orpalladium but does contain nickel. Also SAES showed evidence of fluorineat the top surface of the photoresists. The presence of fluorine ispresumably due to fluorinated surfactants used in these resists toreduce the striations formed during coating (Elliott, MicrolithographyProcess Technology for IC Fabrication, McGraw Hill, N.Y. p. 71 (1986))and adhesion of the resist surfaces to masks during contact exposures.These surfactants make the surface of the resists extremely hydrophobicand are probably required to prevent the metallization of the top of thephotoresist posts.

FIGS. 4-6 show SEM micrographs of microcylinder arrays fabricated onantimony doped silicon wafers. These structures were mounted on copperstubs sputtered with gold and tested for field emission. The emissiontest apparatus and conditions have been described previously(Kirkpatrick et al, App Phys. Lett., vol. 59, p. 2094, (1991)). Emissionfrom structures with a turn on voltage of 300 Kv/cm and a fieldenhancement of ˜300 was achieved with 17 micron tall, 2 micron tipdiameter structures. However, emission from these structures wasdestructive resulting in melting and ejection of the structures from thesurface of the chip. FIG. 7 shows SEMs of the metal microstructuresafter field emission testing.

Characterization of the structures shown in FIGS. 4-7 by XPS depthprofiling showed that thin oxide interfaces were present at the goldnickel and nickel-silicon interfaces. A schematic of the cross sectionof one of the microcylinders, showing the metal and oxide interfaces isillustrated in FIG. 8. Previous work with sharp tips of tantalumsilicide with oxide interfaces on the structures revealed destructivemelting and ejection of the structures from the surface of the chipduring field emission testing. The destructive melting and ejection ofthe microstructures is presumably due to the oxide interfaces which cancause resistive heating at the metal oxide interfaces resulting inmelting and thermal stress.

Example 3 Oxide Free FEA Fabrication and Characterization

In order to eliminate the oxide interfaces in the microcylinder arrays,the following changes in their fabrication was adopted. To make betterelectrical contact to the silicon substrate, the native silicon oxidemust be removed and kept from reforming. This can be accomplished byfirst removing the native oxide with an HF dip and then evaporating ˜500Å of titanium onto the wafer surface which getters any remaining oxide.Next ˜1500 Å of nickel is evaporated onto the titanium in situ. Thenickel acts as a diffusion barrier so that the gold film which followscannot diffuse into the silicon and form a detrimental silicide. Thegold is then evaporated onto the nickel which inhibits oxidation of thenickel and forms a good final electrical contact. The substrate is thenannealed at 350° C. for 15 minutes to accelerate the titanium getteringand to promote interdiffusion of the metal interfaces.

Wafers with the Ti/Ni/Au overcoat described above were then processed asdescribed previously to form 10-12 micron tall microcylinders with 1-3micron wide tip diameters. Immediately following the removal of thephotoresist, the substrate was annealed at 300° C. for 10 minutes topromote diffusion of the nickel, tin, and palladium metals into the goldsurface and to reduce nickel oxide formation on the microcylindersurface.

The arrays were then cut into chips and mounted on copper stubs asdescribed earlier and placed in a combination directional reactive ionetcher/sputtering chamber. The mounted arrays were then etched with anargon or hydrogen etch to remove any nickel oxide and were thensputtered with ˜200 Å of gold. Other mounted arrays went through justthe etching step and then were immediately transferred into the fieldemission test stands.

All microcylinder arrays fabricated using the approach described abovedemonstrated stable DC field enhanced emission with turn on voltages inthe 80-200 Kv/cm and currents in the 50 μA range. The structures showedno evidence of damage even after running for 2 hours DC. Both the goldcoated and uncoated arrays ran, representing the first demonstration ofemission from nickel microstructures. One array was also tested using aphosphorescence screen as the anode which was imaged through a videocamera. This demonstrated that the emission was primarily from the metalmicrostructures and that emission from patterned addressable arrays ispossible. XPS depth profiling of the samples described above, before andafter field emission, showed that no oxide interfaces were present atthe gold-nickel and nickel-gold interfaces. Also arrays that were etchedbut did not receive a gold overcoat showed that the nickel oxide hadbeen removed, but over a period of days, a new oxide will form that isdetrimental to the emission efficiency of the array.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A hollow metal microcylinder consisting ofmetal and having a length of 1 to 200 μm, an outer diameter of 0.25 to30 μm, and an inner diameter of 0.25 to 30 μm.
 2. The microcylinder ofclaim 1, which is open at both ends.
 3. The microcylinder of claim 1,which is closed at one end and open at one end.
 4. The microcylinder ofclaim 1, having a length of from 1 to 30 μm.
 5. The microcylinder ofclaim 1, having an aspect ratio of from 1-200.
 6. The microcylinder ofclaim 1, which is tapered at one end.
 7. The microcylinder of claim 1,having an outer diameter of 0.5 to 13 μm and an inner diameter of 0.5 to13 μm.